In a substantial variety of industrial processes, it is necessary to contact metal surfaces with circulating water in the presence of air. This occurs, for example, in heat transfer water systems where water is used for heating and cooling purposes, e.g. in heat exchangers, cooling towers, chillers, distillation and condensation water systems. In many cases, the water is recirculated from a reservoir over metal surfaces of the apparatus to effect cooling and heating thereof. Since the water is saturated or nearly saturated with oxygen in many cases, the conditions promote corrosion of the metal. Metals commonly used in such apparatus include iron, aluminum, copper and alloys of such metals, two or more of which metals or alloys are normally present in the same apparatus, and all of which are subject to corrosion.
In addition, in such processes, scale deposition is liable to occur on the metal surfaces due to hardness producing cations such as calcium, magnesium and the like dissolved in the water ("hard" water). Such scale deposition inhibits the heat transfer capability of the metal surfaces, and should accordingly be minimized.